One of the central functions of an oil and gas well drilling rig or platform is to handle drill string tubulars or pipes for drilling operations and casing running operations. These are very labour intensive operations, particularly on drilling rigs on land. These are also operations that are fraught with opportunities for the workers to get injured. Statistics show that that a large percentage of the accidents that happen on drilling rigs are associated with handling drill string tubulars.
Traditional pipe handling on drilling rigs or derricks has evolved over many years. Pipe handling methodologies or procedures have been developed around the idea of a very well coordinated drilling crew that learned how to handle pipe in very specific ways using very specific tools and procedures. These procedures have been well established over the years with each crew member having a specific function in the overall process.
A typical pipe handling operation involves retrieving and storing drill string tubulars (and casing) on pipe racks or in pipe tubs located adjacent a drilling rig catwalk. A drill pipe or tubular is usually manually rolled onto the catwalk by two or three workers. If the pipe is inside a pipe tub they are usually raised to the catwalk level by a hydraulic mechanism and rolled from the tub to the catwalk by workers.
A worker wraps a cat line (a simple hoisting line suspended from the derrick) around an end of the pipe and the pipe is then dragged up a v-door into a position straddling the drilling rig floor and the catwalk. From this position, the pipe may remain there or be immediately lifted up and lowered into a “mouse hole”. Once in the mouse hole, the pipe is added to the overall drill string in a procedure known as “making a connection” to increase the length of the drill string. This operation is repeated as necessary.
At different depths of the well, for a variety of reasons, a drill string may be required to be withdrawn in a procedure called “tripping out”. The drill string is hoisted up one segment, or pipe stand, at a time. The pipe stand, which may include multiple joints of pipe, is then “broken off” (disconnected or un-threaded) from the drill string and moved sideways and “racked back” in a racking board (sometimes call monkey board). The racking board is attached to the drilling rig mast itself. The set back area is supported by the substructure. This process is repeated until the entire drill string has been pulled out of the hole. The process may require hundreds of pipe stands to be tripped out and racked back depending on the length of the drill string and the height of the derrick (in single, double or triple stands).
Racking back is usually done manually by workers. Once a pipe stand has been broken off, workers push the bottom end of the stand over to the set back area on the drill floor and carefully lowers a bottom end of the pipe stand onto the floor. The top end of the stand is disconnected from the rig hoisting system and the top end of the stand is moved (manually pulled over by the derrick man) into the racking board and racked between the fingers in the finger board.
The stands must be positioned precisely so that they lean just the right amount to stay where they have been put but not so much that they put an undue side force on the derrick. The whole procedure is reversed for tripping into the hole.
At the end of a drilling operation, when the well has been drilled to total depth (TD), the drill string is tripped out one last time and “laid down”. In this operation, only one single joint at a time (not a multiple joint stand) is pulled out of the hole, broken off and manually and laid down. This is a very time consuming process compared to tripping pipe into the racking board particularly on a big triple rig.
Most pipe handling equipment has been designed to mechanize some small part of the overall procedure. For example, iron roughnecks (power wrenches), for making and breaking of tool joints, were one of the first pieces of equipment to be developed. Other pieces of equipment have been developed to deal with other parts of the job. However, most of the equipment developed were not integrated with each other in an operational way. This is still done by the rig crew who operated each individual piece of equipment in a particular sequence.
Most of the current pipe handling equipment is built to augment, rather than replace, traditional pipe handling procedures. In other words, they do not change the fundamental way pipe is handled. Instead the tools do the same job, the same way a worker would do, except the tool allows the work to be performed faster, better, and safer. This way, the operation does not have to stop if a piece of equipment breaks down, the tool is simple set aside and a worker does the same job manually with manual tools. This redundancy is highly valued in a drilling operation for many reasons.
Many attempts have been made to automate or at least mechanize the handling of drill string tubulars. Most pipe handling systems are made up from several different pieces of equipment that are more or less coordinated with each other. However, as pipe handling requirements on drilling rigs are diverse, not one system has been developed that solves all of the safety and operational issues associated with handling drill string tubulars.
Pipe handling has been difficult to mechanize because of many factors which includes but is not limited to: 1) the diverse ways drill string tubulars or pipes have to be manipulated during various operational procedures; 2) the different types of tubulars a drilling rig has to handle (drill pipe, drill collars, casing, tubing); 3) the different types of downhole tools that have to be handled (DST tools, core barrels, mud motors, stabilizers, shock subs, jars etc); 4) the diverse sizes of tubulars a drilling rig has to be able to handle (2⅜″ to 20″ diameter); 5) the differing lengths of tubulars that have to be manipulated (2 feet to 93 feet); and 6) the differing weights of tubulars (100 lbs to 10,000 lbs) a drilling rig must handle.
As a result of the various requirements for each drilling rigs, most drilling rigs are currently custom built, more or less “fit for purpose”, and intended to do a particular kind of drilling job that limits the range of diversity that the rig and equipment has to handle, making it easier to incorporate some pipe handling equipment into the rig design and mechanize some of the processes. Customization of drilling rigs for a particular job site is expensive and does not allow that customized drilling rig to be used at a different site with ease and without major modifications. The “general purpose” rig, more commonly used in the earlier days of oil and gas drilling, is more capable of handling a wider range of jobs.
The general purpose land rigs are typically divided in three large groups, for the purpose of rig size and depth capacity.
Small rigs, more commonly known as singles, are generally of 50-150 tonne capacity and capable of handling single (30-45 ft) joints of drilling tubulars. These drilling rigs are used to drill shallow wells in the range of 1,000-4,000 ft depth.
Medium rigs, more commonly known as doubles, are generally of 150-250 tonne capacity, capable of handling stands comprising double (60 ft) joints of drill pipe. These are used to drill medium depth wells between 3,000-8,000 feet. The derrick structures are typically taller to accommodate the longer drill string stands. For deeper wells, it is more efficient to have a taller rig with double stands, particularly for tripping operations. It is also necessary to have a taller derrick to rack back more drill string tubulars in the derrick.
Large rigs, known as triples, are generally of 250-750 tonne capacity, capable of handling stands comprising triple (90 ft) joints of drill pipe. These rigs drill deep depth wells between 6,000-30,000 feet. The derrick structures are usually taller then the medium rigs to accommodate the longer drill string stands. These rigs can accommodate even more drill pipe by racking back triple stands and these rigs also have larger floor areas to be able to rack back more stands in the derrick.
The vast differences in rig size and configurations have made it difficult to design a single ubiquitous pipe handling system that fits all sizes of rigs. Instead, two different general design paths for handling drill string tubulars have developed: one for handling drill pipes on single rigs, and one for handling drill pipes for double and triple rigs. The principal difference between these two paths is in the handling of drill string tubulars for tripping operations.
Many mechanized pipe arms have been developed for handling drill string tubulars for single rigs. These pipe arms differ from conventional systems in that instead of having a racking board and storing the drill string tubulars in the derrick for tripping, the pipe stands are picked up or laid down all the time by the pipe arm. The hydraulically powered arm grips pipe stands from the catwalk and lifts the stand directly into position above the wellhead for connection to the drill string. The intermediate steps of placing the stand in the mouse hole and placing the stand in the racking board are eliminated. However, if the hydraulically actuated pipe arm breaks down, the whole drilling process is delayed because workers cannot perform the pipe handling functions in a manual way. There is no V-door, catwalk or mouse hole associated with these types of pipe handling systems. The entire rig is not set up for conventional manual intervention.
These rigs are also usually fitted with top drives and iron roughnecks so that the stands can be spun in, and torqued up, hands free. The stands are never stored in the derrick and thus there is no need for a derrickman. A properly designed single rig with a pipe arm and other automation equipment (such as top drive, hydraulic elevators, link tilt, power wrench, pipe tubs, etc.) represents the most complete pipe handling system available on rigs today. It is also relatively simple.
However, there is a serious limitation with this arm design. It only works well on single rigs. Pipe arms are usually capable of only handling single stands, not the double and triple stands that are in use on bigger rigs. The arms would become too large and heavy if pipe arms are designed for double and triple stands.
The physical geometry of a drilling rig also makes it very difficult to use pipe arms on a high substructure because pipe arms cannot be made to reach up and over a drill floor that is 30-40 feet high. Still, because pipe arms have been so successful, more and more rigs are built as singles and are effectively competing with doubles (and in some case triples) on deeper wells.
For double and triple rigs, automation has been done in smaller discrete steps rather than large complete systems and follows the traditional approach of manually performing many operations with the assistance of mechanical tools. Top drives, power wrenches, pipe spinners have been introduced on these large rigs with good success. Unfortunately, most of the equipment developed for the double and triple rigs has not been integrated into a single system for handling pipes.
Typical double and triple rigs now have top drives, power wrenches, pipe spinners, rotating mouse holes for offline stand building and pipe tubs. These pieces of equipment mechanize certain parts of the pipe handling function but not all and not in an integrated way. The coordination of these separate tools is still done manually by workers who operate them.
More recent advances to the double or triple rigs were the implementation of power catwalks or pipe skates. These automated machines are a combination of the v-door and drilling rig catwalk. Hydraulically powered, power catwalks and pipe skates move the pipe stands from the catwalk position to the v-door. These power catwalks mechanize yet another (small) part of the pipe handling operation as well as assisting in casing running operations by picking up (at the start of the well) and laying down of the drill string (at the end of the well). The power catwalk has no function for tripping drill string since these rigs still rack back the stands in the derrick.
The latest piece of equipment to be introduced on double and triple rigs was the installation of some form of a manipulator arm that can lift a drill string stand from above a centerline of the wellbore and move it to the racking board during tripping out and tripping in operations. The manipulator arm, usually mounted on the racking board, replaces a derrickman and other servicemen on the drill floor and basically trips in and trips out drill stands mechanically.
However, the racking board mounted manipulator arm has some disadvantages. In order to perform any service work on the arm, a worker has to climb up 50-90 feet up in the air and work in a very exposed position. The arm has to be assembled and disassembled for moving the rig.
It is noted that on offshore drilling platforms, sophisticated pipe handling systems have been installed in order to increase operating efficiency and safety. On very large offshore rigs there have been a number of systems designed to mechanize the entire pipe handling process.
Such systems are only possible because the equipment for such systems can be permanently installed on the drilling rigs and do not have to be dismantled, transported on trucks between wells, and then reassembled at a different location, as is the case on land rigs.
The pipe handling systems on the offshore drilling rigs tend to be extremely complicated, large, slow and expensive. The systems require a lot of tuning and maintenance and is only possible on large offshore drilling platforms as these type of rigs usually have technicians, welders, mechanics and electricians on board at all times. It is not practical or economical to install offshore type pipe handling systems on land rigs.
There is still a need for a universal pipe handling system that can be used on most rigs regardless of size and purpose.